CD player

Is it a detachable face unit? If so, there's usually a button behind the face plate that you can press to reset them. If not, remove the positive side battery terminal for a couple of minutes? Maybe some needle nose pliers and a hammer?
 
I'm not entirely sure. I think you may have to remove the head unit completely and see if there's some sort of hole you can shove a paper clip into (that's what most aftermarket head units have). That forces the CD to be ejected manually.